The use of carbon/carbon composite substrate materials is widespread in modern industry, particularly in the aerospace and aviation fields. However, it is well known that such carbon/carbon composite materials are relatively susceptible to oxidation at elevated temperatures. For this reason, it has been found desirable to provide these carbon/carbon composite materials with a primary protective coating in order to minimize the occurrence of oxidation of the carbon/carbon composite material at elevated temperatures.
Reinforced carbon/carbon materials, formed from graphite fabric impregnated with phenolic resin, have been provided with an oxidation resistant silicon carbide coating and used as the thermal protection system for the wing leading edge and nose cap surfaces of a Space Shuttle Orbiter. The oxidation resistant coating was formed by blending 60 wt % silicon carbide, 30 wt % silicon and 10 wt % alumina powders, and packing this mix around the carbon/carbon substrate in a graphite retort. Then the retort and its contents were heated to 3000.degree. F. in an argon atmosphere. During the heating process, the outer layers of the carbon/carbon substrate were converted to silicon carbide. The silicon carbide coated substrates were then removed from the retort and cleaned.
Other examples of a primary protective coating for a carbonaceous substrate material are disclosed in U.S. Pat. Nos. 4,585,675 and 4,830,919 to Shuford. These Shuford patents disclose a protective coating for a carbonaceous substrate wherein the protective coating comprises about 40% to about 50% by weight silicon, about 30% to about 50% by weight silicon carbide, and about 20% to about 30% by weight alumina. U.S. patent application Ser. No. 638,045, filed Aug. 6, 1984 by Shuford, now U.S. Pat. No. 5,453,324, discloses a protective coating for a carbonaceous substrate wherein the protective coating is formed by first applying to the substrate a first mixture comprising particulate silicon, particulate silicon carbide, and particulate alumina, heat treating the substrate having the first mixture thereon, then applying to the thus treated substrate a second mixture of particulate silicon, particulate silicon carbide, and particulate boron, and then heat treating the substrate having the second mixture thereon.
Primary coatings of the type used in the Space Shuttle Orbiter and of the type disclosed by Shuford tend to have a high coefficient of expansion relative to the carbonaceous substrate to which they are applied. As a result of the disparity between the coefficients of expansion of the primary coating and the carbonaceous substrate, cracks in the primary coating tend to develop during cycles of heating and cooling, thereby exposing the carbonaceous substrate to oxygen.
Various efforts have been made to overcome the above referenced development of cracks in the primary protective coatings. For example, U.S. Pat. Nos. 4,585,675 and 4,830,919 to Shuford and U.S. patent application Ser. No. 06/638,045 by Shuford, now U.S. Pat. No. 5,453,324, disclose the use of an enhancement coating formed by impregnating the silicon carbide/boron carbide primary protective coating with tetraethyl orthosilicate. The tetraethyl orthosilicate enhancement coating can be heat cured at a temperature of approximately 3000.degree. F. in order to form a silica coating on the carbonaceous substrate. Shuford further discloses the subsequent application of a mixture of a liquid alkali silicate and a silicon carbide powder over the tetraethyl orthosilicate enhancement coating.
Silicon sealants of the type disclosed by Shuford are typically molten at both low and high temperatures, thereby enabling them to flow into the cracks in the primary coating as such cracks develop. However, due to the fact that these silicon sealants are molten throughout a wide range of temperatures, they tend to be forced out of the cracks in the primary coating as the cracks close with increasing temperature. At least a portion of the silicon sealant thus forced from the cracks may be effectively removed from the surface of the coated carbon/carbon material during normal use, thereby precluding that portion of the silicon sealant from flowing back into cracks in the primary protective coating as such cracks reopen at lower temperatures. This effect is particularly prevalent when the carbon/carbon material is subjected to numerous heating and cooling cycles during which cracks in the primary coating repeatedly form and close.
Gray, U.S. Pat. No. 4,894,286, discloses incorporating a mixture of silicon, titanium, and boron metals into a carbon/carbon matrix in order to provide oxidation protection for the carbon/carbon matrix at high temperatures. In an example, a prepregging resin was prepared with 65 parts by weight of a phenolic resin, 10 parts by weight alcohol, and 35 parts by weight of a glass precursor powder. The glass precursor powder comprised 24.54 wt % Ti.sub.5 Si.sub.3 (-325 mesh), 10.64 wt % SiC (-600 mesh), 24.67 wt % SiB.sub.6 (-325 mesh), and 40.15 wt % B (sub-micron). The prepregging resin was incorporated into plies of thermally stabilized satin fabric, and the resulting laminate was laid up, cured, carbonized and densified. The thus prepared substrate was CVD coated with a silicon rich silicon carbide. The patentee concluded that two samples which had the SiSiC coatings exhibited no weight loss for 327 and 470 hours when subjected to a defined thermal test cycle, while a third sample exhibited a weight loss after an initial gain.
Weir et al, U.S. Pat. No. 4,931,413, disclose the use of a glass ceramic precursor composition to protect graphite, carbon, ceramic, and metals such as low carbon steel, from oxidation at elevated temperatures. The composition can comprise titanium diboride, a silica compound such as colloidal silica, and optionally an intermetallic compound such as silicon carbide, boron carbide and titanium carbide. A preferred composition is described as comprising 35 wt % titanium diboride, 40 wt % colloidal silica, and 25 wt % silicon carbide. The patentee indicated that for a coating composition, the particle size should be less than 80 mesh and preferably less than 200 mesh. The patentee also indicated that care had to be exercised in the firing cycle to prevent the coating from popping off the host material.
However, as the physical and chemical characteristics of various forms of carbon vary greatly, a material which may provide oxidation protection for one type of carbon will not necessarily provide the desired degree of protection for a different type of carbon. Similarly, a protective composition containing many of the same elements as another protective composition will not necessarily perform as effectively as the other protective composition for a particular carbon form substrate.